The Myler Flat Shank with Sweet Iron Ported Barrel (MB 33, Level 3), made from sweet iron with copper mouth inlay, is a great choice for a horse considered broke and who obeys commands, is relaxed at the poll, and has advanced skills. The generously curved mouthpiece affords maximum relief to your horse's tongue. 
Details
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The Basics: Sweet iron with copper inlay. Sweet iron will oxidize naturally over time, is harmless to horses, and its sweet taste encourages salivation.
\nLevel Up: Myler's unique "level" system means there's a bit for your horse no matter their training. Level 3 bits offer maximum tongue relief and work using bar, poll, and curb pressure with little to no tongue pressure. Ideal for experienced, trustworthy, and consistent horses with mild to no control issues.
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Benefits of a Flat Shank: Flat shanks are curb bits that require less contact than other bits. They are well-suited for advanced, dexterous riders who use soft hands with horses who respond well to light bit pressure. 
\nIndependent Side Movement: Entire shank moves independently from the mouthpiece which means more control and clearer communication between you and your horse.